Due to increasing demand, processing of fresh produce, in particular lettuce, has expanded significantly over recent years. The harvesting and processing of lettuce involves extensive cutting of the leaves, which induces a strong wound response. This wound response leads to a rapid deterioration of the processed product. This deterioration is manifested by discolouration due to enzymatic browning or pinking at and around the wound surface, respiration and desiccation due to transpiration. The enzymatic browning or pinking is considered of significant importance in determining directly or indirectly the overall quality of the fresh cut, packaged lettuce.
Moreover, as a consequence of the deterioration, micro-organisms can significantly increase in number, which may compromise food safety. The highly perishable nature of processed lettuce leads to a strong off-colour, off-odour and off-texture perception by the consumer, which has hampered faster growth of the so called convenience market.
In order to inhibit the deterioration process, many chemical or physical post-harvest treatments have been developed, which can be applied to decelerate the deterioration of the processed lettuce. Amongst these are the packaging of fresh cut lettuce under a modified atmosphere, application of edible coatings, heat shock treatment, and addition of chemicals which inhibit the enzymatic browning. When fresh cut lettuce is packaged under an atmosphere of reduced oxygen at low temperatures, the enzymatic browning can be substantially reduced. However such modified, low oxygen environment leads to anaerobic respiration, which creates an off-flavour and off-odour of the produce that is perceived as very unattractive.
Edible coatings are thin layers of materials, which act as a physical insulation barrier and which effectively protect the produce from different forms of deterioration such as evaporation and browning. These coatings can for example be made of resins, polysaccharides or protein.
It has further been demonstrated that browning of fresh cut lettuce can be prevented by applying a brief a heat shock of 90 seconds at 45° C., immediately after processing. Possibly, the heat shock diverts protein biosynthesis from the enzymes involved in discolouration towards heat shock proteins thereby reducing the enzymatic browning capacity. Alternatively, the effect of heat shock treatment on browning may be explained by thermosensitivity of enzymes involved in the discolouration pathway.
Chemicals which can be applied can be, for example, reducing agents like vitamin C, chelating agents like EDTA, complexing agents like cyclodextrin and enzymatic inhibitors like L-cysteine. Application of chemicals in fresh food obviously involves food safety issues and requires regulatory approval. Combinations of the post-harvest technologies described above can be thought of and ultimately the applied procedure is a trade-off between technological efficacy, cost and food safety.
Irrespective of the technology applied, improvement of post-harvest quality of processed lettuce will come at a cost; therefore, a clear need in the art exists to provide alternatives that eliminate or reduce the need to apply physical or chemical post-harvest technologies.